dossier on the world's most mysterious leader. Until shortly before he took power 15 months ago, this was the only known photograph of kim jong un, a snapshot of a boy just 11 years old. We still debate how old he is, we think perhaps 30, but i still see quotes as 28 or 29. Rchlt very little is known about someone whose finger is on a nuclear button. Reporter: But here's what we do know, he and his older brother are believed to have studied under fake names at this private high school in switzerland. Former classmates have offered a window into who he is, describing kim jong un they knew as a calculating leader on the basketball court. Jong un would call them together, would point out the successes or the shortcomings of the players. Reporter: The family's private sushi chef has said the father, after drinking heavily one night, dismissed his oldest son, as, quote, girlish and said of kim jong-un, that boy is like me. Interestingly, now kim jong-un seems to be imitating his grandfather, north korea's father. Notice the hairstyle, the hat, and like his grandfather, he gives big public speeches. In contrast, his father is only known to have uttered two sentences in public. Ever. The grandfather gained fame and power by fighting the japanese in world war ii. Now kim jong-un is taking on a bigger superpower, even threatening a nuclear attack against the united states. But like his grandfather, also a brutal dictator, the grandson is believed to be coldly calculating, but not crazy. How stable is he? How volatile is he? I think he's dangerous but not unstable. I'm not concerned that north korea will launch a single nuclear warhead against washington. They would know that's national suicide. Reporter: White house officials believe the threats are more tactical than a real move towards war. Nobody knows for sure. Jonathan karl, abc news, the white house.

This transcript has been automatically generated and may not be 100% accurate.

North Korea's a 'Real and Clear Danger'

Secretary of Defense Chuck Hagel addresses threats made by North Korea.